Prior art dump trucks and trailers have a problem when discharging their loads by raising the front end of the box to slide the contents out the rear by gravity. An extension is needed to carry the discharging material farther from the rear edge of the truck or trailer box but that means either shortening the truck or trailer box itself to stay within the highway length restrictions or doing without such extension from the rear edge of the box. The solution is the rotatable spreader pan or extension member in accordance with this invention, which can be pivoted to its extension position for unloading and then pivoted downward out of the way after unloading so the overall length of the truck or trailer is no greater than without an extension and is still within the highway limits.